Material preview mode transparency (Blender 2.8 Eevee)
Solution 01
Add a reference image

Blender can add Reference images, that are visible in the 3D viewport, but not in renders.
They're great choice for modelling, and have many options available.
Add > Image > Reference


Solution 02
Enable transparency in Eevee

To make a material appear transparent in Material preview, use the Alpha value.
Don't forget to set Blend mode to Alpha-blend.
(If you cannot find these settings, switch to Eevee render engine in the render tab)